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A little clarification on
metataxa.....
David Marjanovic
wrote-
>What is a
metataxon?
Then I wrote-
>A taxon people are too lazy to find apomorphies
of, or else a nomen dubium they really don't >want to drop. ;-) Okay,
seriously, it's a taxon with no apomorphies, but which can be >distinguished
from other taxa of the same time period. I strongly disagree with this
concept, as >I feel people think of apomorphies only as these obvious new
characters and not as >proportional or shape differences, which are just as
valid in my opinion. The only instance I can >see where a metataxon
might be appropriate is if some elements were named that were >absolutely
identical in morphology to another species, but this "metataxon" was
over thirty >million years older/younger than the other species and
preferrably on another continent (though >even that has the paleogeography
thing to cancel it out).
This was all based on the way Oliver uses metataxa
in his thesis. He defines them as such, and I stand by what I said
above. I then wrote-
>Some people think ancestors of species are
metataxa and that some fossil specimens are >actually ancestors. This
may work well for well sampled groups like sharks or mammals where >we can
see one form grading into another through time, but the dinosaurian record is
far less >complete. Any hypothetical ancestor is going to be different
from the main stock it's evolving >from, as it is developing mutations that
will make it a new species. If these differences cannot >be seen in the
fossil (which is very possible, as so little anatomy is preserved), it should be
>classified as a member of whatever it evolved from. This is obvious
and probably done quite >often, although we would never know. If the
differences can be seen in the fossil, then use the >differences to define it
and make it a new species. Whatever it eventually evolved into can be
>defined as another species as it has yet more characters that developed
after its ancestor. >This is all hypothetical and probably useless at
the present, as dinosaur taxonomy and >diversity is too poorly known to be
worrying about species concepts yet.
This is based on a message from Oliver, but I
looked in the archives and found Tom Holtz has conveniently posted the
definition of a metataxon at least three times for the list-
"Although the chance of finding fossils of the population directly
ancestral to later species is very small, a cladist can recognize a potential
ancestor as one which:
a) shares derived features with the hypothesized descendant;
b) lacks derived features unique to itself;
c) is found stratigraphically lower than the descendants; and (it would be
nice)
d) is found in a region later inhabited by the descendants.
Any fossil which qualifies for aspects a and b is called a "metataxon".
Thus, potential ancestors are metataxa which occur earlier than the hypothesized
descendants."
So a metataxon is technically a species that shares
synapomorphies with its descendent, but hasn't developed apomorphies of its
own. I see how this could happen, but don't think any dinosaurian examples
have been found yet.
For instance, Darren Naish has suggested
Aristosuchus is a metataxon (http://www.cmnh.org/fun/dinosaur-archive/2000Nov/msg00142.html) . Just what is the hypothesized descendent of
Aristosuchus? Certainly not Compsognathus, which lived thirty million
years earlier. Perhaps the still unnamed Santana compsognathid, SMNK 2349
PAL? It lived only ten million years later, albeit in Brazil. Of
course, this specimen is also called a metataxon by Darren. In fact, if we
assume Darren is correct that Aristosuchus is identical to Compsognathus, it
can't share identifiable synapomorphies with its descendent that aren't present
in Compsognathus. So it can't be a metataxon. It would be a nomen
dubium. This is an example of my first type of metataxon- "a nomen dubium
they really don't want to drop".
For the other type of metataxon, lets use
Archaeopteryx. I've heard this taxon used often as a metataxon, as Tom
says- "Archaeopteryx occurs earlier than any other confirmed birds, it possess a
few derived features shared with later birds but lacking in other theropods, and
lacks any derived features of its own." Archaeopteryx would be a metataxon
if it had no apomorphies. But we have seven specimens, several complete,
to deal with. Surely there's an apomorphy somewhere. For instance-
no neural spines on caudal vertebrae; manual phalanges III-1 and III-2 sutured;
narrow tapered distal tip of ischium separated from obturator process by distal
notch. These are just the characters I noticed while casually glancing at
some papers. Who knows how many would be found if someone actually
looked. Most of Oliver's metataxa are surely of this type- "A taxon people
are too lazy to find apomorphies of". So when you see in Oliver's thesis
that Coelophysis, Piatnitzkysaurus or Dryptosaurus is a metataxon, I urge you to
look for apomorphies.
Mickey Mortimer
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